1. Field of the Invention
The present invention relates to an electrostatic discharge protective circuit and a semiconductor integrated circuit device using the same.
2. Description of the Related Art
As an interface of a semiconductor integrated circuit device is made to function at higher speeds, there is increasing demand for higher speed input/output buffer circuits connected between an internal circuit and an input/output terminal of a semiconductor integrated circuit. Normally, a logic circuit section of the internal circuit is required to be operated particularly at high speeds, and as a result a thin film transistor in which a thin gate insulation film is used is provided as a component thereof. Meanwhile, the input/output buffer circuit is formed of a thick film transistor in which the gate insulation film is formed to be thick, but in order to operate the input/output buffer circuit at high speeds, in some cases, the input/output buffer circuits also need to be formed of thin film transistors.
For example, as shown in FIG. 7, an output buffer circuit 76 which is formed of a pair of thin film PMOS device 71 and NMOS device 72 is connected between power source terminals 74 and 75 to which voltages VDD and VSS are applied, respectively. These terminals 74 and 75 will be referred to as VDD terminal and VSS terminal, hereinafter. An output of the output buffer circuit 76 is supplied to an I/O terminal 73. An ESD (electrostatic discharge) protective circuit 79 which comprises the PMOS device 77 and the NMOS device 78 is provided in parallel with the output buffer circuit 76. In this case, the gate of the NMOS device 78 which forms the protective circuit 79 is connected to the power source terminal 75 such that the NMOS device 78 is normally set to be off and is not turned on due to an output signal applied to the I/O terminal 73.
In this state, when the ESD voltage is applied to the I/O terminal 73, and if the gate of the thin transistors 71 and 72 of the output buffer circuit 76 is in a floating state, the output buffer circuit 76 will be turned on first. As a result, a large ESD surge current flows from the NMOS device 72 of the output buffer circuit 76 to the VSS terminal 75, and the NMOS device 72 may be damaged before it is protected by the ESD protective circuit 79.
In addition, the size of the ESD protective circuit 79 must be increased in order to increase the current capacity of the discharge path, but in general, the parasitic capacitance is large for a large MOS device, and this is inconsistent with increasing the operation speed. For this reason, as shown in FIG. 8, instead of using MOS devices in the ESD protective circuit 79, a protective device 81 such as an SCR (Semiconductor Controlled Rectifier) which has low parasitic capacitance and high discharge capacity is used in the ESD protective circuit together with diodes 82 and 83. In the circuit of FIG. 8, the VDD terminal 74 and the I/O terminal 73 are isolated by the diode 82 and the I/O terminal 73 and the VSS terminal 75 are isolated by the diode 83.
In the case where the protective device 81 comprising the SCR is used as the ESD protective circuit, the trigger voltage of the SCR must be set so as to be lower than the trigger voltage or the turning on voltage of the NMOS device 72 of the buffer circuit 76. However, since the electric potential at the gate of the buffer circuit and the internal circuit shown in FIGS. 7 and 8 at the time when the ESD voltage is applied is not generally fixed and is almost in a floating state, so that it is difficult to predict the sufficient trigger voltage. For example, it is impossible to accurately set the trigger voltage for the MOS device 72 at all cases. As a result, it is also difficult to set the trigger voltage at the protective device 81.
An example of a prior art using such an ESD protective circuit comprising SCR is that described Jpn. Pat. Appln. KOKAI Publication No. 8-293583. In the technology described in this publication, SCR is used for protecting an input/output buffer, but the configuration is complex because an SCR trigger dedicated circuit for triggering the SCR is formed separately from the input/output buffer. In addition, when ESD voltage is applied, a buffer circuit that is to be protected before the SCR trigger dedicate circuit operates, operates first and thus there is the possibility that a large current caused by the ESD flows to the buffer circuit which is damage.